1. Technical Field
The disclosure relates to RF-driven plasma sources for reactors employed in plasma processing of workpieces such as semiconductor wafers.
2. Background Discussion
In plasma processing of workpiece, such as a semiconductor wafer, there is a need for a plasma source capable of providing a high plasma ion density and, simultaneously, a low plasma sheath ion energy to an extent that is currently unavailable. A high plasma ion density is needed for improved processing rate and productivity. A reduced plasma ion energy is needed for reduced plasma ion energy in order to prevent contamination from ion bombardment of metal surfaces near the plasma sheath. Reduced ion energy may also reduce ion bombardment damage to semiconductor device features. Such features are becoming extremely small and more susceptible to such damage, thus requiring reduction in plasma electron energy.
A basic problem is that plasma sources capable of providing high density plasma also produce relatively high energy plasma ions. The reason is that such sources couple relatively high electric fields to the plasma, raising the plasma sheath voltage. High plasma sheath voltages impart high energy to plasma ions in the plasma sheath. This produces ion bombardment of metal surfaces adjacent the plasma sheath, which produces metal contamination. An inductively coupled plasma source employs an RF-driven coil antenna, which has a capacitance that couples a high voltage to the plasma, contributing to the high plasma sheath voltage. A capacitively coupled plasma source employs an RF-driven electrode which has an even greater tendency to couple high voltage to the plasma. Toroidal plasma sources produce plasma densities somewhat less than inductively coupled plasma sources.
What is needed is a plasma source capable of producing a plasma having an ion density as great as or exceeding that of a conventional inductively coupled plasma source, and with a minimum plasma ion energy less than (or not exceeding) that of conventional plasma sources.